An Apical Cell in the Shoot Apex of Isoetes tuckermanii

Abstract

Well defined apical cells in the meristems of pteridophytes other than Isoetes typically have a simple geometric shape, a relatively large size and nucleus, and a high degree of vacuolation. In radially symmetrical apices, the apical cell is located at the center of the apex. But the most important requirement of an apical cell is that it be the source from which all cell lineages in the apex are derived. The recent history of segmentation of an apical cell may be deduced from the geometrical relationships and relative thickness among the walls of the surrounding cells (Bierhorst, 1977). The most recently formed cell walls are the thinnest; older walls are thicker due to the deposition of additional primary wall material as cells become subdivided into groups or packets of cells. The shoot apex of Isoetes was originally described by Hofmeister (1862) as having a single apical cell which was the ultimate origin of all other cells in the shoot. Using various species including the one which Hofmeister studied (I. lacustris), later workers have either denied the existence of a single apical cell (Bruchmann, 1874, Hegelmaier, 1874; West & Takeda, 1915; Bhambie, 1957) or have allowed that at least in some cases such a cell may be present (Scott & Hill, 1900; Lang, 1915; Paolillo, 1963). Scott and Hill suggested that Hofmeister may have been influenced in his interpretation by the fact that the apical cell was the only type of apical organization known at that time, but they were careful to point out that occasionally an apical cell may be present. Scott and Hill found several examples of what they thought might be apical cells, but they could not determine if the large size and central position of these cells were stable or if these properties would have been lost in the course of further activity of the meristem. The present report confirms the existence of an apical cell at least in some specimens of I. tuckermanii A. Br.